Method of wafer plating

ABSTRACT

Provided is a technique for wafer plating treatment that ensures that the plating film thickness becomes uniform on the total area of a plated wafer surface. This method of wafer plating includes: arranging a wafer in an opening of a plating tank; bringing a peripheral side of the wafer and a cathode electrode into contact with each other; supplying a plating liquid; causing the plating liquid that has reached the wafer to flow in the direction of a periphery of a wafer surface to be plated; and supplying a plating current by an anode electrode arranged within the plating tank so as to be opposed to the wafer and the cathode electrode, whereby the wafer is subjected to a plating treatment. In this method, the anode electrode has a shape almost the same shape as the wafer surface to be plated, a plurality of peripheral-edge current supplying sections are provided in a peripheral edge of the anode electrode, and a central current supplying section is provided in a center of the anode electrode. A peripheral-edge plating current supplied from the peripheral-edge current supplying sections and a central plating current supplied from the central current supplying section can be adjusted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plating treatment of a wafer for semiconductors.

2. Description of the Related Art

In performing a plating treatment of a wafer for semiconductors, there has hitherto been known a method of wafer plating that involves arranging a wafer in contact with a cathode electrode in an opening of a plating tank, supplying a plating liquid to this wafer, causing the plating liquid that has reached the wafer to flow in the direction of a periphery of a wafer surface to be plated, supplying a plating current by an anode electrode, being arranged within the plating tank so as to be opposed to the wafer, and the cathode electrode, whereby the wafer is subjected to a plating treatment. This method of wafer plating is widely used as a method suitable for small-lot production and for automation of the plating treatment process, because the plating treatment can be performed sequentially by replacement of the wafer for the opening of the plating tank. In this plating treatment of a wafer, it is important that the film thickness be uniform on the whole area of a plated wafer surface and various improvements have been proposed in order to improve uniformity of the film thickness.

For example, in a case where it is impossible to realize a uniform film thickness on the whole area of the plated wafer surface due to a local concentration of a plating current, there is known a measure to mitigate the concentration of a plating current by disposing a shielding plate in accordance with the variation in film thickness and the like in the plating tank (refer to Japanese Patent Application Laid-Open No. 8-74088). Also, as a measure to be taken on the cathode electrode side, there is known a measure that involves dividing a cathode electrode that is brought into contact with a peripheral edge of a wafer and forming a current mirror circuit by the divided cathode electrode and an anode electrode so that uniform electrodeposition is performed on the whole area of a surface being plated. Refer to Japanese Patent Application Laid-Open No. 2001-115297. Furthermore, on the anode electrode side, there is known a method that involves dividing an anode electrode disposed opposite to a wafer into a plurality of parts, i.e., a peripheral anode electrode and a central anode electrode so that the two have mutually insulating regions, and controlling the coating thickness by reducing an energization time for the plating treatment to the central anode electrode compared to the energization time for the plating treatment to the peripheral anode electrode. Refer to Japanese Patent Application Laid-Open No. 7-197299. According to these conventional techniques, it is possible to perform a plating treatment on a wafer surface to be plated resulting in a film thickness having a certain level of uniformity.

However, taking measures such as arranging a shielding plate in the plating tank and dividing the anode electrode makes the construction of the plating apparatus complicated. And each time when the wafer diameter changes, it is necessary to make an adjustment suited to the diameter, i.e., a size change of the shielding plate, an adjustment of the shape of a divided anode electrodes and the like. Furthermore, dividing the anode electrode and individually controlling divided portions of the anode electrode, has disadvantages in cost, such as the need to prepare a plurality of rectifiers. For this reason, in performing the plating treatment of wafers, there are required plating treatment techniques capable of realizing a uniform film thickness with a simpler construction of a plating apparatus and with simpler measures including the maintenance of the plating apparatus.

In the measure that involves supplying a plating current by use of a divided cathode electrode so that uniform electrodeposition can be performed on a wafer, the measure is not sufficient for the film thickness control on the whole area of a plated surface, including the center of the wafer, although the plating treatment in the peripheral portion of the wafer is controllable.

Moreover, in the recent semiconductor industry, the trend is toward larger-diameter wafers and for example, wafers 12 inches, i.e. approximately 300 mm, in diameter have made their appearance. Under the present circumstances, therefore, plating treatment techniques easily realizing a more uniform film thickness on the whole area of a plated surface are much demanded for such large-diameter wafers.

Therefore, the present invention is intended for providing a method of wafer plating capable of performing plating with a uniform film thickness on the whole area of a plated surface even for a large-diameter wafer by improving conventional wafer plating treatment techniques.

SUMMARY OF THE INVENTION

To solve the above-described problem, the present invention provides a method of wafer plating including: arranging a wafer onto an opening of a plating tank; bringing a peripheral side of the wafer and a cathode electrode into contact with each other; supplying a plating liquid; causing the plating liquid that has reached the wafer to flow in the direction of a periphery of a wafer surface to be plated; and supplying a plating current by an anode electrode arranged within the plating tank so as to be opposed to the wafer and the cathode electrode, whereby the wafer is subjected to a plating treatment. In this method of wafer plating, the anode electrode has a shape almost the same shape as the wafer surface to be plated, a plurality of peripheral-edge current supplying sections are provided in a peripheral edge of the anode electrode, a central current supplying section is provided in a center of the anode electrode, and a peripheral-edge plating current supplied from the peripheral-edge current supplying sections and a central plating current supplied from the central current supplying section are adjusted. According to this plating method, it is unnecessary to arrange a shielding plate in the plating tank or to modify the plating apparatus into a special structure and it becomes possible to perform plating treatment with a uniform film thickness on the whole area of a plated wafer surface.

It is desirable that the plurality of peripheral-edge current supplying sections in the method of wafer plating of the invention be provided in at least three places, more preferably in at least four places. If the number of the peripheral-edge current supplying sections is too large, the effect of the peripheral-edge current supplying sections contributing to the uniformity of a film thickness tends to decrease. In consideration of large-diameter wafers to be subjected to a plating treatment, it is desirable that the peripheral-edge current supplying sections be provided in not more than ten places. Practically, it is preferred that the peripheral-edge current supplying sections be provided in four to eight places.

The adjustment of a peripheral-edge plating current in the anode electrode and a central plating current supplied from the central current supplying section in the method of wafer plating of the invention is to distribute a total plating current. That is, a plating current of a prescribed proportion in the total plating current supplied to the anode electrode, is supplied to the central current supplying section, and a plating current corresponding to the remainder is allocated to the peripheral-edge current supplying sections. For example, in a case where an area near the center of the wafer surface to be plated is subjected to a plating treatment with a larger film thickness than the peripheral part, 40% of the total plating current is supplied to the central current supplying section and the remaining 60% is allotted to the peripheral-edge current supplying sections. This proportion for plating current adjustment can be appropriately determined in accordance with the coating thickness condition that has become nonuniform. For the adjustment of the current to be supplied, it is possible to adopt, for instance a method by which the current value itself is set so that a current supplied to the center and a current supplied to the periphery have different values from each other, a method by which the plating treatment time, i.e. energizing time, is made different between the center and the periphery.

In the method of wafer plating of the present invention, it is preferred to adjust the plating current distribution by using the anode electrode formed of a flat plate and by providing a plurality of bores in the flat plate. When a plurality of bores are made in the anode electrode formed of a flat plate, the plating liquid passes through the bores. By adjusting the number of the bores and the places where the bores are formed, it becomes possible to adjust the plating current distribution on the wafer surface to be plated. Plating treatment with a more uniform film thickness becomes possible by adjusting the supply of a plating current to the anode electrode and simultaneously adjusting the plating current distribution.

And in the method of wafer plating of the present invention, it is preferred to adopt an anode electrode in which a platinum film as an intermediate layer is formed on an electrode substrate made of titanium and an iridium oxide film is formed on a surface of the intermediate layer. When an anode electrode of such a construction is used, it is possible to improve the uniformity of the plating thickness while suppressing a substantial increase in electrode costs, and the corrosion resistance to the plating liquid is also excellent, with the result that it is possible to maintain the stability of the plating solution, namely maintain the property that does not destruct the plating solution in an electrolytic plating treatment. In particular, when a wafer is subjected to a gold plating treatment, with an anode electrode of such a construction, the deposition of gold does not occur on the electrode surface even in the case of a gold plating treatment for a long period of time and therefore the maintenance of the electrode becomes easy.

As described above, according to the present invention, even in the case of a large-diameter wafer, it is possible to perform a plating treatment with a uniform film thickness on the whole area of a plated wafer surface without the need to substantially change the construction of the plating apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a cup-shaped plating apparatus in the first embodiment;

FIG. 2 is an enlarged schematic plan view of an anode electrode;

FIG. 3 is a plan view that shows film thickness measuring points on a wafer surface to be plated; and

FIG. 4 is an enlarged schematic plan view of an anode electrode used in the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

An embodiment of the present invention is described below. FIG. 1 schematically shows a cross-section of a plating tank of a cup-shaped plating apparatus in this embodiment. As shown in FIG. 1, the cup-shaped plating apparatus 1 of this embodiment is constructed in such a manner that a wafer W can be placed along an opening of a plating tank 2, and a ring-shaped cathode electrode C is arranged so as to come into contact with a peripheral part of the wafer W. Under the cathode electrode C, there is arranged a seal packing 4 for preventing leakage of a plating liquid.

In the plating tank 2, a plating liquid supply port 5 is provided in the middle of the bottom, and a plating liquid outflow port 6 is provided to ensure that the plating liquid supplied as an upward flow toward the wafer W placed on the opening 2 can flow out to the outside of the plating tank 2. Furthermore, on the bottom side of the plating tank 2 is provided with an anode electrode A so as to be opposed to the placed wafer W.

FIG. 2 shows an enlarged schematic plan view of the anode electrode A shown in FIG. 1. The anode electrode A is provided with a plurality of bores 10, through which the plating liquid can flow. In this anode electrode A was used an electrode material whose surface was coated with iridium oxide (IrO₂), which was obtained by plating a disk-shaped electrode substrate made of Ti with Pt (film thickness: 0.5 to 2 μm), further plating the Pt film with Ir (film thickness: 1 to 2 μm), and thereafter performing a heat treatment in the room-air atmosphere by use of an electric furnace (refer to Japanese Patent Application Laid-Open No. 2006-22379).

Furthermore, the anode electrode A is provided with peripheral-edge current supplying terminals (ar1 to ar4) in four places of a peripheral edge thereof and a central current supplying terminal (ac) in the center thereof, each of these terminals connected to a not-shown plating current supplying power source. Incidentally, for the ring-shaped cathode electrode C, connection terminals provided in four places at the periphery of the electrode were connected to the plating current supplying power source.

Next, a description is given of results of an evaluation test of wafer plating treatment conducted by use of the cup-shaped plating apparatus in this embodiment described above. This test was conducted by using a wafer with a seed metal obtained by forming a TiW film (3000 Å) on a surface of a wafer to be plated, which has a diameter of 8 inches (approximately 200 mm), and forming a seed metal film of Au (1000 Å) on the TiW film surface. For the anode electrode A, a disk made of Ti having a diameter of 204 mm and a thickness of 1 mm was plated with Pt and Ir and after heat treatment, bores having a diameter of 8 mm were formed in 161 places uniformly on the whole surface of the electrode (refer to Japanese Patent Application Laid-Open No. 2006-22379).

The plating treatment was performed by using a non-cyanide based, weakly alkaline high-purity gold plating liquid (made by Japan Electroplating Engineers Co., Ltd., product name: MICOFAB Au660). The plating treatment using this gold plating liquid was performed under the plating conditions: liquid temperature 60° C., pH 7 to 8 and plating current density 0.8 A/dm², and a film thickness of 18 μm as the target plating thickness. The surface of the seed metal film of the wafer was coated with a resist in a thickness of 25 μm, and a bump forming pattern (total aperture area: 0.35 dm²) used in forming a plurality of bumps for liquid crystal in the form of rectangular columns was formed on the resist and subjected to a plating treatment. And the plating current supply amount was regulated so that the proportion of the peripheral-edge plating current value (the total current value of the four terminals ar1 to ar4) and to the central plating current value in the anode electrode during the plating treatment becomes 3:7. This proportion of the supply amount was determined beforehand by fabricating a wafer subjected to a plating treatment by setting the supply amount for the central plating current value and the peripheral-edge plating current becomes uniform in the whole anode electrode, checking the coating thickness of the wafer, and comparing the coating thickness between the vicinity of the center and the peripheral edge part. The adjustment of the proportion of the plating current supply amount between the peripheral-edge plating current value (the total current value of the four terminals ar1 to ar4) and the middle-par plating current value was performed by first supplying only the central plating current for a prescribed time and thereafter supplying the peripheral-edge current for a prescribed time, with the middle-par plating current not supplied.

To make a comparison, a gold plating treatment was performed by using a mesh anode electrode made of Ti (an expand metal having a diameter of 204 mm, a rhombic opening having a major axis of 11 mm and a minor axis of approximately 8 mm) plated with Pt (thickness: 4 μm) in the cup-shaped plating apparatus shown in FIG. 1. In this mesh anode electrode, a current supplying terminal was provided in one place of its peripheral, i.e. a position corresponding to ar1 in FIG. 2, and the plating current was supplied from this terminal in one place to the anode electrode. Other plating treatment conditions were the same as described above.

The evaluation of the plating treatment test was performed by exfoliating the resist of the wafer after plating treatment and measuring the height, i.e. coating thickness, of the bumps in the form of rectangular columns. This bump height, i.e. coating thickness, was measured by use of a stylus type profiler (KLA-Tencor P11). Concretely, for the bumps in the form of rectangular columns formed at the center of the wafer surface to be plated and in the periphery thereof as shown in FIG. 3, the bump height in a total of 13 places (W1 to W13) was measured. Table 1 shows the average value (Avg.), maximum value (MAX.), minimum value (MIN.), dispersion range (Range.) and dispersion range (Range.)/average value (Avg.) obtained from results of this bump height measurement. Incidentally, the unit of the average value (Avg.), maximum value (MAX.), minimum value (MIN.) and dispersion range (Range.) shown in Table 1 is μm, and the unit of the dispersion range (Range.)/average value (Avg.) is %.

TABLE 1 Avg. MAX. MIN. Range. Range./Avg. Working 18.06 18.97 17.42 1.54 8.5 example Comparative 18.72 21.73 16.65 5.09 27.2 example

In the working example shown in Table 1, the plating current supply was performed from the four places in the peripheral edge and center of the anode electrode, whereas in the comparative example, the plating current was supplied from one place of the peripheral edge of the mesh anode electrode. From the results shown in the table, it became apparent that the difference between the maximum value and the minimum value, i.e., the dispersion range (Range.) in the working example is by far smaller than in the comparative example. The Range./Avg. value, which is obtained by dividing the dispersion range by the average value) is clearly small in the working example, and it was ascertained that this plating treatment has very high uniformity in terms of the coating thickness.

In the comparative example, the plating treatment was performed in such a manner that the film thickness of a spot of the plated wafer surface corresponding to the current supplying terminal portion provided in one place of the anode electrode became extremely large, and it became apparent that this phenomenon increases the dispersion range of the coating thickness. In contrast to this, it became apparent that in the case of the working example, the plating treatment had been performed so that also the coating thickness in the vicinity of the periphery of the plated wafer surface became uniform. In this working example, the current supply on the cathode electrode side was not divided. However, by dividing the current supply on the cathode electrode side, concretely, by combining a method of divided supply on the cathode electrode side as disclosed in Japanese Patent Application Laid-Open No. 20001-115297 described above, and a method of divided supply on the anode electrode side of this working example upon supplying the plating current, the supply of the plating current is controlled, whereby it is possible to improve the uniformity of the coating thickness on the whole plated wafer surface, particularly, the uniformity of the coating thickness in the vicinity of the periphery of the plated wafer surface.

Second Embodiment

Subsequently, another embodiment of the present invention is described. In this second embodiment, a cup-shaped plating apparatus of the same construction as in the first embodiment above (FIG. 1) was used. However, this plating apparatus of the second embodiment can treat wafers 12 inches, i.e. approximately 300 mm in diameter. For the anode electrode A used, a disk made of Ti having a diameter of 294 mm and a thickness of 2 mm was plated with Pt and Ir and after heat treatment, bores having a diameter of 10 mm were formed in 361 places uniformly on the whole surface of the electrode. The manufacturing method of the anode electrode A is the same as the above-described first embodiment. As shown in FIG. 4, this anode electrode A is provided with peripheral-edge current supplying terminals (ATR, AR, ABR, ATL, AI, ABL) in six places of a peripheral edge thereof at equal intervals and a central current supplying terminal (AC) in the center thereof, each of these terminals connected to a not-shown plating current supplying power source. Incidentally, for the ring-shaped cathode electrode C, connection terminals provided in seven places at the periphery of the ring-shaped cathode electrode C were connected to the plating current supplying power source. The distance between the anode electrode A and the opposed wafer, i.e., the anode-cathode distance was set at 21 mm.

Next, a description will be given of results of an evaluation test of wafer plating treatment conducted by use of the cup-shaped plating apparatus in this embodiment described above. This test was conducted by using a wafer with a seed metal obtained by forming a TiW film (3000 Å) on a surface of a wafer to be plated, which has a diameter of 12 inches, i.e. approximately 300 mm, and forming a seed metal film of Au (1000 Å) on the TiW film surface. Gold bumps of a specified shape were formed on the surface and the uniformity of plating treatment was checked by measuring the height of the formed gold bumps. In this test, two kinds of gold bumps were formed and each of the gold bumps was evaluated. One has the shape of a rectangular column, and a plurality of bumps of this kind having a target bump height of 23 μm were formed on the surface of a wafer, more specifically the total bump-formed area on the wafer surface is approximately 0.1 dm², called the bump <A>. The other has also the shape of a rectangular column, and a plurality of bumps of this kind having a target bump height of 16 μm were formed on the surface of a wafer, more specifically the total bump-formed area on the wafer surface is approximately 1.0 dm², called the bump <B>. Product specifications require that these two kinds of bumps be formed with a deviation of within 2 μm from the target bump height (that the difference between the maximum value and the minimum value within all bumps whose height is measured be within 2 μm.

In the gold plating treatment for performing bump formation, the same gold plating liquid as in the first embodiment was used. For the plating conditions, the liquid temperature was 60° C., the pH value was 7 to 8 and plating current density was 0.8 A/dm². The surface of the seed metal film of the wafer was coated with a resist in a prescribed thickness corresponding to each bump height, a bump forming pattern for forming a plurality of bumps in the shape of a rectangular column was formed on the resist, and gold plating treatment was performed thereafter.

The plating current supply to the anode electrode during the plating treatment was performed by methods as described below. Concretely, the formation of the gold bumps was performed by the following four kinds of plating current supplying methods: a method by which the plating current is supplied from all of the six places at the periphery of the anode electrode shown in FIG. 4 (the first method), a method by which the plating current is supplied from one place of ATL shown in FIG. 4 (the second method), a method by which the plating current is supplied from the two places of ABR and ABL shown in FIG. 4 (the third method), and a method by which the plating current is supplied from one place of the center of the anode electrode (the fourth method).

The evaluation of the plating treatment test was performed in the same way as in the first embodiment by exfoliating the resist after the plating treatment and measuring the height, i.e. coating thickness of the bumps in the shape of a rectangular column. The bump height, i.e. coating thickness was measured by use of a stylus type profiler (KLA-Tencor P11). Concretely, for the bumps in the form of rectangular columns formed at the center of the wafer surface to be plated and in the periphery of the wafer as shown in FIG. 3, the bump heights in a total of 13 places (W1 to W13) were measured. Table 2 shows the average value (Avg.), maximum value (MAX.), minimum value (MIN.), and dispersion range (Range.) obtained from results of this bump height measurement. Incidentally, the unit of the average value (Avg.), maximum value (MAX.), minimum value (MIN.) and dispersion range (Range.) shown in Table 2 is μm.

TABLE 2 Bump Supply method Avg. MIN. Range. <A> First method 24.49 23.76 1.87 Second method 24.37 23.33 2.25 Third method 24.63 23.71 2.00 Fourth method 23.73 22.78 2.56 <B> First method 16.19 15.48 1.15 Second method 16.18 15.61 1.51 Third method 16.17 15.76 1.25 Fourth method 16.03 15.54 0.99

As shown in Table 2, when the bumps <A> are to be formed, it became apparent that the first method or the third method can form gold bumps capable of meeting the dispersion range of 2 μm required by the product specifications according to the plating current supply. On the other hand, in the case of the bumps <B>, under all of the first to fourth methods, it was possible to form gold bumps capable of meeting the dispersion range of 2 μm required by the product specifications. The results shown in Table 2 reveal different tendencies between the bumps <A> and the bumps <B>. However, it can be thought that this is because in the formation of the bumps <A>, the total plated area is by far smaller than in the bumps <B>. 

1. A method of wafer plating comprising the steps of: arranging a wafer on an opening of a plating tank; bringing a peripheral side of the wafer and a cathode electrode into contact with each other; supplying a plating liquid; causing the plating liquid that has reached the wafer to flow in the direction of a periphery of a wafer surface to be plated; and supplying a plating current by an anode electrode arranged within the plating tank so as to be opposed to the wafer and the cathode electrode, whereby the wafer is subjected to a plating treatment, wherein the anode electrode has a shape almost the same shape as the wafer surface to be plated, a plurality of peripheral-edge current supplying sections are provided along a peripheral edge of the anode electrode, and a central current supplying section is provided in a center of the anode electrode, and wherein a peripheral-edge plating current supplied from the peripheral-edge current supplying sections and a central plating current supplied from the central current supplying section are adjusted.
 2. The method of wafer plating claimed in claim 1, wherein the anode electrode is formed from a flat plate and the plating current distribution is adjusted by providing a plurality of bores in the flat plate.
 3. The method of wafer plating claimed in claim 1 wherein the anode electrode is configured such that a platinum film as an intermediate layer is provided on an electrode substrate made of titanium and an iridium oxide film is provided on a surface of the intermediate layer.
 4. The method of wafer plating claimed in claim 2 wherein the anode electrode is configured such that a platinum film as an intermediate layer is provided on an electrode substrate made of titanium and an iridium oxide film is provided on a surface of the intermediate layer.
 5. The method of wafer plating claimed in claim 1 wherein the anode electrode comprises from three to ten peripheral-edge current supplying sections.
 6. The method of wafer plating claimed in claim 1 wherein the anode electrode comprises from four to eight peripheral-edge current supplying sections.
 7. The method of wafer plating claimed in claim 1 wherein 40% of the total plating current is supplied from the central current supplying section and 60% of the total plating current is supplied from the peripheral-edge current supplying sections.
 8. The method of wafer plating claimed in claim 1 wherein the plating liquid comprises gold.
 9. The method of wafer plating claimed in claim 1 wherein the anode electrode comprises a central current supplying terminal and four peripheral-edge current supplying terminals.
 10. The method of wafer plating claimed in claim 1 wherein the anode electrode comprises a central current supplying terminal and six peripheral-edge current supplying terminals.
 11. The method of wafer plating claimed in claim 1 wherein the cathode electrode is ring shaped and comprises four peripheral-edge current supplying terminals.
 12. The method of wafer plating claimed in claim 1 wherein the cathode electrode is ring shaped and comprises seven peripheral-edge current supplying terminals. 